As is well-known, a dry film is used for forming resists for wiring circuit formation in the production of print wiring boards. This dry film is made by at least forming a photosensitive resin layer in a semidried state on a support film.
In a method of using the dry film, first, the photosensitive resin layer is laminated on a material to be treated (substrate) with adding heat. Subsequently, a photo tool is attached firmly onto a support film surface of the photosensitive resin layer, and an area to be cured is cured by exposing with ultraviolet light. Then, after peeling the support film, an unexposed area is removed by developing with a weak alkali aqueous solution to obtain a circuit pattern of a resist. Recently, high-density print wiring substrates have been progressively developed, and thus high sensitivity and high resolution have been required in the dry films used for the production of the print wiring substrates.
A photosensitive resin composition which forms the aforementioned photosensitive resin layer contains a polymer component (A), a photopolymerizable monomer component (B) and a photopolymerization initiator (C) as essential components. As the photosensitive resin composition for the aforementioned dry film, for example, a constitution using an alkali-soluble resin as the polymer component (A), using a common crosslinking compound as the monomer component (B), and using the photopolymerization initiator (C) at 0.1 to 15 parts by weight based on a total 100 parts by weight of the alkali-soluble resin and the crosslinking compound, and using a multifunctional thiol compound (D) having two or more mercapto groups in one molecule at 0.1 to 15 parts by weight based on the total 100 parts by weight of the alkali-soluble resin and the crosslinking compound has been disclosed (e.g., Patent Document 1).
Meanwhile, a laser direct imaging (LDI) technology which requires no photo tool has attracted a lot of attention because many kind of products in small amounts may be efficiently produced with a high degree of accuracy. The laser direct imaging (LDI) technology has also attracted a lot of attention because a production process of the print wiring boards is shortened by omitting production steps because of using no photo mask and it is not necessary to bother the contaminated photo mask. Thus, the suitable photosensitive resin composition obtained using this laser direct imaging (LDI) technology has also been proposed (e.g., Patent Document 2).
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-220409 A
Patent Document 2: Republished WO 01/71428
In laser scanning exposure performed in the above laser direct imaging (LDI) technology, it is preferable that sensitivity of resists is higher in order to increase productivity of the print wiring board because a scanning speed is dependent on the sensitivity of a photosensitive resin composition. However, there is generally a trade-off relationship that stability for light and heat becomes poor when the sensitivity is enhanced in the photosensitive resin composition. Thus, a sensitivity property has been changed by the safety light such as yellow light and the heat generated in the production process of the print wiring board thereby generating a problem of limiting a workability. Therefore, photosensitive resin compositions whose stability for the light and the heat is enhanced with maintaining the high sensitivity has been desired. The photosensitive resin composition disclosed in the above Patent Document 1 has been insufficient in terms of this stability.
A copper through-hole method (tenting method) for obtaining a print wiring plate by forming a circuit pattern of the resist as described above, subsequently etching a copper surface, and peeling a cured resist by a strong alkali aqueous solution such as sodium hydroxide has been a mainstream in methods for producing the print wiring plate because the production process is simple and is easily managed compared with a plating method.
In the pattern formation by this tenting method, higher resolution has been required, and it has been described that it is necessary to thin a photosensitive resin layer of a dry film in order to realize the high resolution. However, when the photosensitive resin layer of the dry film is thinned, there has been a trouble in that no sufficient tent strength is obtained and thus the resist is broken by a developing solution and a washing solution.
That is, in the conventional photosensitive resin compositions, both the sensitivity and the stability for the light and the heat have been seldom satisfied, and further the tent strength and the resolution have been seldom sufficient.